Microwave filter using hybrid circuit and resonators

ABSTRACT

A first one of the two ports of a first pair of conjugate ports of a directional coupler forms the input of the filter. Two line sections, one short-circuited, the other open, are respectively coupled to the conjugate ports of the second pair. Two resonators tuned to a frequency Fo are respectively coupled in parallel to the two line sections at a distance of d/8 (d: wavelength corresponding to the frequency Fo) of the ends of these two line sections opposite those coupled to the directional coupler. According to whether the frequency of an input signal is Fo or not, this signal is collected at the second port of the first pair, or returned to the input port.

United States Patent [1 1 Destrade et al.

[ June 24, 1975 1 MICROWAVE FILTER USING HYBRID CIRCUIT AND RESONATORS 175] Inventors: Michel Destrade; Jean-Luc Vassort,

both of Paris, France Cohn 333/10 X Hoovler 333/10 X Primary E.taminerPaul L. Gensler Attorney, Agent, or Firm-Cushman, Darby & Cushman [57 1 ABSTRACT A first one of the two ports of a first pair of conjugate ports of a directional coupler forms the input of the filter. Two line sections, one short-circuited, the other open, are respectively coupled to the conjugate ports of the second pair. Two resonators tuned to a frequency F are respectively coupled in parallel to the two line sections at a distance of d/8 (d: wavelength corresponding to the frequency F of the ends of these two line sections opposite those coupled to the directional coupler. According to whether the frequency of an input signal is F,, or not, this signal is collected at the second port of the first pair, or returned to the input port.

4 Claims, 2 Drawing Figures PATENTEDJUN24 ms i 3891.948

HYBRID URCUIT ,/5

MICROWAVE FILTER USING HYBRID CIRCUIT AND RESONATORS The object of the present invention is a branching filter intended notably to equip multiplex arrangements operating with metric and decimetric waves.

The branching filters used at high frequencies are filters with the property of operating, with regard to the signals they receive, by transmission or by reflection, depending on the frequency of the signals. Whether they are in the form of a coaxial or a waveguide structure, they are generally composed of several resonators with a high quality factor 0, arranged in a cascade and spaced at an interval of d/4 (d being the wavelength corresponding to a frequency of the band to be transmitted by the filter) along one and the same transmission channel.

Design of resonators with a high quality factor necessitates the use of complicated and expensive technical solutions in the choice of materials, finishing and surface treatments and in the choice of methods of producing moving tuning parts.

Furthermore, in power equipment, heating due to losses in the resonators causes mechanical deformations which give rise to frequency drifts; this makes it necessary to use auxiliary devices to stabilise the tuning of the resonators.

The aim of the invention is to overcome the foregoing drawbacks by the use of a new method of coupling the resonators, enabling the use of resonators with a lower quality factor and receiving a lower power.

According to the invention, there is provided a branching filter comprising: a hybrid circuit having two pairs of conjugate ports; two line sections having respective first ends connected respectively to the ports of one and the same pair of conjugate ports and respective second ends, one of which is open and the other short-circuited; and two resonators tuned to the same frequency F and respectively coupled in parallel to said two line sections at the same distance from said second ends.

The invention will be better understood and other characteristics will appear from a consideration of the following description and the related drawings in which:

FIG. 1 illustrates the principle of a branching filter according to the invention;

FIG. 2 is the view of a form of construction of a branching filter according to the invention.

FIG. 1 shows a branching filter and a load resistance Z, connected to this filter. The branching filter comprises:

a hybrid circuit 5 whose four ports carry the references l, 2 for one of the pairs of conjugate ports and 3, 4 for the other pair of conjugate ports and have a characteristic impedance 2,;

a line section 8, one end of which is connected to the port I and the other end of which is short-circuited, and a resonator 6, schematically shown in section, tuned to a frequency F coupled in parallel to the line section 8 at a point A (the frequency F, is chosen in the band of the frequencies to be transmitted by the filter; it may, for example, be taken equal to the geometrical mean of the limiting frequencies of this band);

a line section 9 of the same length as the line section 8, one end of which is connected to the port 2 and the other end of which is open, and a resonator 7, schematically shown in section, tuned to the frequency F,,, coupled in parallel to the line section 9 at a point B,

The load resistance Z is connected to the input 4. The line sections 8 and 9 have a characteristic imped- 5 ance Z, and a length d/l between their point of coupling to the resonator and their end opposite to the hybrid circuit 5 (where d is the wavelength corresponding to the frequency F Since the load resistance of the resonators is Z,,, their tuning frequency F and their quality factor under load 0 (taking into account 2 and the coupling with the line section), the admittance of each resonator is, for a frequency F, in the form where x F:-

The impedance applied to the port 1 by the resonator 6 and the line section 8 in parallel is:

Z 1r with r and 0 The impedance applied to the port 2 by the resonator 7 and the line section 9 in parallel is:

strated that for F equal or close to the tuning frequency, F, the ratio P /P of the power P;, applied to the port 3 to the power P, effectively transmitted to the port 4 (ratio defining the tapping losses of the filter) assumes the form:

The tapping losses of a classic filter consisting of two resonators coupled in a cascade in one and the same transmission channel are of the form 1+1/4 Q') (where Q is the quality factor common to the resonators), so it must be noted that to obtain the same response characteristic, i.e. the same tapping losses as a function of the frequency, it is sufficient to use in the filter of FIG. I resonators which have a quality factor 0 equal to half the quality factor Q of the resonators in the corresponding conventional filter.

It must also be noted that in relation to a conventional filter each of the resonators receives only half of the power to be transmitted. Finally, the presence of the line sections introduces an additional control parameter which is the length of these sections: the response characteristic of the filter may be adjusted without altering the tuning of the resonators through making d different from the value of the wavelength corresponding to F FIG. 2 is a view of a form of construction of the branching filter, the principle of which is given in FIG. 1. In this form of construction the line sections have a coaxial structure.

The ports 1 and 2 (corresponding to the ports 1 and 2 in FIG. 1) of a hybrid circuit, which is in this example a 3- decibel directional coupler, 50, are coupled respectively by two coaxial line portions 81, 91 to two resonators 60 and 70. These resonators are likewise coupled respectively to two coaxial line portions 80 and 90. The line portion 80 terminates at its end opposite to the resonator 60 in a metal plate 82 creating a short circuit between its external conductor and its in ternal conductor. The line portion 90 is extended at its end opposite to the resonator 70 by a metal cylinder 92 with the same diameter as its external conductor; the purpose of this metal cylinder 92 is to prevent any parasitic radiation due to the line portion 90.

To show clearly the structure of the resonators and their junction with the line sections, one of them 60 and the corresponding coaxial line portions 80 and 81 have been assumed to be transparent; since it is understood that the two resonators are identical, the description of the resonator 70 would be the same as that of the resonator 60.

The resonator 60 comprises a hollow metal cylinder 61, the bottom of which is closed by a metal plate 62 and, inside this cylinder, a tuning piston 21 and coupling means 25, 26 for coupling the resonator 60 to the line portions 80 and 81.

In the example described the line portions are portions of a coaxial line with a characteristic impedance of 50 ohms. The coupling means 25, 26 are intended to form a junction with a characteristic impedance of 50 ohms between the line portions 80 and 81. The coupling means comprise a metal disc 25 supported on two insulating legs, the two insulating legs being fixed to the metal bottom 62 of the cylinder 60. The metal disc is circular, its plane is perpendicular at its centre to the axis of the cylinder 60 and its diameter is less than the inside diameter of this cylinder. The internal conductors of the line portions 80, 81 are in along one and the same straight line and they cross without contact the cylinder 61 and are fixed at two diametrically opposed points of the metal disc 25; the external conductors of these line portions are joined to the edges of the holes permitting the corresponding internal conductors to cross the cylinder 61.

The length of the line section in FIG. 1, which is equal to d/8, is, in the form of construction in FIG. 2, approximately that contained betwen the centre of the disc 25 and the short-circuited end of the line portion 80.

The coupling means likewise comprises, under the disc 25, a metal plate 26 fixed by a metal leg to the bottom 62 of the cylinder 60; this plate is intended to increase the capacitive coupling between the disc 25 and the inside surface of the wall of the resonator 60.

The piston 21 comprises a disc 22; this disc is perforated in the middle by a hole and it bears on its circumference against the inside wall of the cylinder 61. The piston 21 likewise comprises a plunger element consisting of two hollow cylinders whose axes coincide with the axis of the cylinder 61; one of these cylinders 23 is joined to the disc 22 which it traverses through the hole which the latter has in its middlei the other one of these cylinders 24 is arranged so as to be able to slide on the inside of the cylinder 23 and its end directed towards the disc 25 is closed by a semi-spherical metal cap, the concavity of which is turned towards the inside of the cylinder 24. The greater or lesser penetration of the disc 22 into the cylinder 61 controls the tuning frequency of the resonator 60 and the greater or lesser penetration of the cylinder 24 into the cylinder 23 controls the coupling between the resonator 60 and the line section composed of the line portions 80, 81 and the junction between these portions; naturally, these two controls react to one another and must therefore be carried out jointly. The mechanical device permitting the control of these penetrations comprises three threaded rods 31, 32, 33 whose depth of penetration into the cylinder 61 is obtained by screwing them into a plate 34 perpendicular to the axis of the cylinder 61 and joined to this cylinder. The disc 22 is suspended from the ends of the rods 31 and 32 in such a way that these rods can be screwed freely into the plate 34; the cap of the cylinder 24 is suspended in the same way from one end of the rod 33. To prevent mistuning of the resonator due to the expansion of the rods 31, 32, 33 under the effect of heating during operation, these rods are made of nickel steel with a very low thermal coefficient of expansion.

By way of example, it has been possible with filters such as the one described above to produce multiplex transmitters admitting a power of 10 kW per channel and operating in a frequency band around lOO mc/s with a frequency difference between two channels which can be reduced to 1.6 mcls and 0.3 decibel of losses due to the tapping of the filters. The filters used in these multiplex transmitters are around I meter high and 20 cm in diameter.

Naturally, the filter according to the invention can be made with, as line sections, waveguides or flat lines; likewise, the resonators can be of a known type.

Furthermore, the lengths between the points A and B and the ends of the line sections 8 and 9 opposite to the hybrid circuit 5 (FIG. 1), which have been assumed to be equal to d/8, can also be chosen in the form (2k+l d/B where k is a positive integer.

As regards the hybrid circuit 5, it may be composed of any hybrid circuit of known type; but, to take into account modifications in the lengths of the four channels (between the ports inside these circuits) in relation to the lengths of the four channels of a S-decibel coupler, FIGS. 1 and 2 must be modified by introducing a difference between the lengths of the line sections located between the hybrid circuit and the resonators, said difference needing to be d/4 in the majority of these hybrid circuits.

Of course, the invention is not limited to the embodiments described and shown which were given solely by way of example.

What is claimed is:

1. A branching filter comprising: a hybrid circuit having two pairs of conjugate ports; two line sections having respective first ends connected respectively to the ports of one and the same pair of conjugate ports and respective second ends one of which is open and the other short-circuited; and two resonators tuned to the same frequency F, and respectively coupled in parallel to said two line sections at the same distance from said second ends.

2. A branching filter as claimed in claim I, wherein said distance is equal to (Zk-l-l) d/8, where k is a non negative integer and where d is the wavelength corresponding to the frequency F 3. A branching filter as claimed in claim 1, wherein said hybrid circuit is a 3-decibel directional coupler and wherein said two line sections have the same length.

4. A branching filter as claimed in claim 1, wherein each of said resonators has a wall provided with two holes located opposite each other, wherein each of said two line sections comprises two portions of a coaxial line, the two portions of a line section being respec- 

1. A branching filter comprising: a hybrid circuit having two pairs of conjugate ports; two line sections having respective first ends connected respectively to the ports of one and the same pair of conjugate ports and respective second ends one of which is open and the other short-circuited; and two resonators tuned to the same frequency Fo and respectively coupled in parallel to said two line sections at the same distance from said second ends.
 2. A branching filter as claimed in claim 1, wherein said distance is equal to (2k+1) d/8, where k is a non negative integer and where d is the wavelength corresponding to the frequency Fo.
 3. A branching filter as claimed in claim 1, wherein said hybrid circuit is a 3-decibel directional coupler and wherein said two line sections have the same length.
 4. A branching filter as claimed in claim 1, wherein each of said resonators has a wall provided with two holes located opposite each other, wherein each of said two line sections comprises two portions of a coaxial line, the two portions of a line section being respectively located in front of the two holes of the resonators coupled to this line section and having respective external conductors connected to the wall of this same resonator and respective internal conductors respectively passing through the two holes of the wall of this same resonator, and wherein said filter comprises two coupling means, respectively located inside said two resonators, for coupling said line sections to said resonators, said two internal conductors of each line section being connected together through the corresponding coupling means. 